1. Field of the Invention
The present invention generally relates to a method of hunting for a communication channel and a device operating based on the method, and particularly relates to a method of hunting for a communication channel and a device operating based on the method where the method hunts for a PCM communication channel in a communication system comprised of a plurality of PCM communication lines.
2. Description of the Related Art
FIG. 1 is an illustrative drawing showing a network access device and a switch in a communication system. FIG. 2 is an illustrative drawing showing two switches in a communication system.
In FIG. 1 and FIG. 2, a switch 10 includes PCM-communication-line control devices 11, a network switch 12, subscriber circuits 13, a signal processing device 15, a central controller 16, a main memory 17, and a bus 18.
In FIG. 1, each of the PCM-communication-line control devices 11 is connected to a network access device 21 via a PCM communication line 20. In FIG. 2, each of the PCM-communication-line control devices 11 is connected to another switch 22. The subscriber circuits 13 are directly connected to subscriber terminals 14. Each PCM communication line 20 has a capacity for 30 different communication channels, for example. In the configuration of FIG. 1, the network access device 21 transmits a call-transmission signal or a call-disconnection signal to the switch 10 via a PCM communication line 20 when such a signal is received from a subscriber terminal connected to the network access device 21. The switch 10 attends to control of the call.
Under the control of the central controller 16, the network switch 12 switches communication lines. The signal processing device 15 receives a call-transmission signal or a call-disconnection signal from a subscriber terminal 14 or from the network access device 21, and searches for or hunts for an available channel in a PCM communication line 20. If a channel is available, the signal processing device 15 allocates the channel to the communication channel of a subscriber who initiated the call. If all the channels are busy in a given PCM communication line 20, a search is conducted with respect to another PCM communication line 20. Then, an available channel that is found is allocated to the communication channel of the subscriber who initiated the call.
When a subscriber initiates a call, there may be a communication channel in a PCM communication line 20 where the communication channel was selected on a previous occasion in response to the subscriber""s previous call. In such a case, a search for an available channel is started from a channel next following the previously selected communication channel in this PCM communication line 20. If an available channel is found, the signal processing device 15 allocates the channel to the communication channel of the subscriber who initiated the call. If all the channels are busy in a given PCM communication line 20, a search is conducted with respect to another PCM communication line 20. Then, an available channel that is found is allocated to the communication channel of a subscriber who initiated the call.
For the purpose of hunting for a channel in an environment where a plurality of PCM communication lines are provided, typically, channel statuses as to whether the channels are available are controlled separately for each PCM communication line. When a subscriber initiates a call, a search for an available channel is conducted in a PCM communication line. If an available channel is found, the channel is allocated to the communication channel of the subscriber who initiated the call. If all the channels are busy in this PCM communication line, a search is conducted in another PCM communication line. When an available channel is found, the channel is allocated to the communication channel of a subscriber who initiated the call. Alternately, an initial search for an available channel may be started from a channel next following a previously selected channel that was chosen for a previous call.
In such a channel hunt method as described above, a channel of a second PCM communication line is allocated in response to a call when all the channels of the first PCM communication line are busy or all the channels next following the previously selected channel in the first PCM communication line are busy. Because of this, a PCM communication line which is currently allocated to calling subscribers tends to have more busy channels than other PCM communication lines. If this PCM communication line suffers failure, many numbers of calls are terminated.
Further, when a PCM communication line is to be shut down for the maintenance purpose, shutting down is not performed until all the channels become non-busy. It takes time to shut down a PCM communication line that is allocated to calling subscribers since such a line has a larger number of busy channels than other lines.
Accordingly, there is a need for a channel hunt method and a device based on such a method which can reduce the number of calls to be terminated at the time of failure of a PCM communication line, and can shorten a time period required for shutting down a PCM communication line.
Accordingly, it is a general object of the present invention to provide a channel hunt method and a device based on such a method which can satisfy the need described above.
It is another and more specific object of the present invention to provide a channel hunt method and a device based on such a method which can reduce the number of calls to be terminated at the time of failure of a PCM communication line, and can shorten a time period required for shutting down a PCM communication line.
In order to achieve the above objects according to the present invention, a method of hunting for a communication channel in a communication system where a plurality of communication lines each have a plurality of communication channels includes the steps of maintaining a record of current channel statuses of the communication channels with respect to all the communication lines, and distributing busy channels evenly among the communication lines by selecting a communication channel based on the record of channel statuses and assigning the selected communication channel to a call.
The method as described above distributes busy channels evenly among the communication lines, so that concentration of busy channels in a particular communication line can be avoided. This reduces the number of calls that are to be terminated at a time of communication-line failure, and shortens a time period required for closing a communication line.
In order to distribute busy channels evenly among the communication lines, according to one aspect of the present invention, the communication channels have channel numbers assigned thereto, and the communication lines have line numbers assigned thereto, wherein the step of distributing the busy channels includes the steps of searching through the record for an available channel having a search channel number by checking the communication lines in an ascending order of the line numbers, repeating the search by successively incrementing the search channel number from a smallest channel number to a largest channel number until the available channel is found, and assigning the available channel to the call.
According to the present invention, there are at least three other variations of the communication-channel-hunt method that achieves even distribution of busy lines. These three variations and the particular variation that was described above may be provided as one set, and a selection may be made among the four variations to perform a search of a desired type to look for a communication channel in a desired manner.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.